Ion kinetic energy spectrometer

ABSTRACT

A spectrometer is provided for analyzing metastable decompositions which occur in certain ions. Metastable decompositions result in a precursor ion decomposing into a daughter ion and an uncharged particle of mass. The presence of such ions determine a unique spectra for certain compounds which are difficult to distinguish using conventional mass spectrometer techniques. The spectrometer utilizes a wedge-shaped beam afforded by the use of a relatively inexpensive electrostatic lens assembly. A drift space is provided to allow precursor ions to decompose and thence be analyzed.

United States Patent [1 1 Delany Oct. 30, 1973 ION KINETIC ENERGYSPECTROMETER [75] Inventor: Edward B. Delany, Ridgefield,

Conn.

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

[22] Filed: Dec. 14, 1972 211 Appl. No.: 315,117

[52] US. Cl. 250/290, 250/283, 250/293, 250/294, 250/427 [51] Int. Cl.H01] 39/34, Bold 59/44 v [58] Field of Search 250/281, 282-, 283, 1

[56] References Cited UNITED STATES PATENTS 3,475,604 10/1969 Noda etal. 250/281 X 3,610,921 10/1971 Major, Jr. 250/283 3,673,404 6/1972Major, Jr. 250/282 Primary Examiner-William F. Lindquist Attorney-JohnK. Conant [57] ABSTRACT 10 Claims, 3 Drawing Figures I Mimi/7151.5

ION KINETIC ENERGY SFECTROMETER This invention relates to spectrometersand more particularly to a spectrometer for performing metastableanaylsis.

BACKGROUND OF INVENTION tation of the mass spectra.

In any event, a common characteristic of many of these instruments isthat ions enter the deflection fields with substantially the same energyor' velocity. After the ionization process of the sample material, awell known phenomenon occurs which is generally referred to as ametastable reaction. Certain sample ions produced, exhibit metastablecharacteristics. In this manner, a parent or precursor metastable iondecomposes into a charged daughter ion and an uncharged particle ofmass. The kinetic energy of an accelerated precursor ion then becomesdistributed among these resultant particles in accordance with the massof the particles. The decomposition of the precursor ion may occur atdifferent points along the trajectory of the ion. In a massspectrometer, when a decomposition occurs at a location between the ionsource and the analyzing field of the spectrometer, the division of theprecursor energy between particles of different masses results in adaughter ion having less kinetic energy then is necessary for focusingat the appropriate sector focal point. Accordingly, it has been foundthat a double focused mass spectrometer, when operated at apre-established electric field potential, will not indicate daughterions as they will be defocused. However, the analysis of metastablecharacteristics can be afforded by the use of a double focused massspectrometer which is altered in structure and'operation.

The analysis of such ion decompositions is useful to the analyticalchemist since it contributes to an understanding of the molecularstructure of many sample materials. Therefore, some analytical chemistshave modified double focusing mass spectrometers to perform ion kineticenergy analysis to determine the existence and identity of daughter andprecursor ions. Such spectrometers are extremely expensive and themodifications to perform such analysis are relatively difficult andinconvenient to implement. The fact remains that metastable analysis isimportant in that certain materials cannot be conveniently detected withthe use of conventional and expensive mass spectrometer techniques. Forexample, in regard to certain isomers, it is extremely difficult todetermine which isomer of a plurality of such isomers is present by atypical mass spectrometer analysis. This is so because of the fact thatthese isomers exhibit almost identical peaks and defy simple detection.However, their characteristics can be accurately ascertained by the useof metastable analysis as will be further explained.

It would therefore be desirable to provide an economical apparatus tospecifically analyze metastable decompositions in order to provideinformation in regard to the constituents of such samples.

BRIEF DESCRIPTION OF FIGURES FIG. 1 is a partial schematic and blockdiagram of an ion kinetic energy spectrometer.

FIG. 2is a schematic diagram ofa beam path used in a spectrometer usinga 30 energy analyzer section.

FIG. 3 is a schematic diagram of another arrangethem using a 30 energyanalyzer section.

DETAILED DESCRIPTION OF DRAWINGS Referring to FIG. 1 there is shown ablock diagram of anion kinetic energy spectrometer. Such a spectrometercan also be utilized in conjunction with a gas chromatograph to provideanalysis of gaseous samples. Briefly, the system comrises an analyzerportion which includes a scanner 50, various power supplies and anelectrometer 60. In order to clarify explanation and description, theoptical system and the electronic system will be separately discussed.

TI-IE OPTICAL SYSTEM Before proceeding with a detailed description ofthe optical system, a few general points should be made. In regard tothe selection of an optical system for an ion kinetic spectrometer, oneis concerned with simplicity and sensitivity rather than highresolution. These criterion involve consideration of the inherentspectral line widths of many samples. It would be reasonable tofabricate an analyzer which could provide a resolution of 200 to 300lines with reasonably sized apertures or slits. An energy analyzer 20using parallel plates with 45 incidence is shown as part of the opticalsystem. Both the entrance S1, 16 and collector slit S 17 can be fixedprior to installation or, alternatively, can be adjustable dependingupon particular system requirements. The ion source 10 is of a generallyopen construction to allow increased helium flow and uses a large areaPierce-type filament to enhance sensitivity. There are included in theion a series of repeller plates 15 to enhance metastable ion productionin the drift space. The lens system is designed to match the source ofion formation to the entrance slit 16 of the energy analyzer 20. In oneinstance, the lens allowed a 10 inch drift path in a field-free spacewhile providing a very small beam divergence.

relatively non-critical tolerances of lens parts and spacing thusachieving an economical, advantage in the overall systemcharacteristics. A quadrapole doublet lens is a very efficient way toget a wedge-shaped beam together with a long drift length which isnecessary to give the precursor ions time to decompose into a daughterion and a charged particle. This therefore provides an efficient andeconomical lens assembly for such a spectrometer.

A wedge-shaped beam can also be produced by a magnetic quadrapole lensassembly. However, magnetic quadrapoles are expensive and one does notrequire a magnetic lens to form a wedge-shaped beam. In this manner, theuse of the electrostatic quadrapole assembly provides a convenient wayof producing an optimum beam shape resulting in relatively sharpfocusing of the beam. The sharper the focusing of the beam, the moreimproved the sensitivity, as this provides a larger current densityacross the entrance slit 16 of the energy analyzer section 20.

THE ELECTRONIC SYSTEM The ion source includes a cathode or filament andan anode electrode or target. The filament geometry is a Pierce-typefilament which is a large filament and hence gives greater sensitivityand as such is sometimes referred to as a space charged limited ionsource. Electrons are emitted from the cathode and drawn to the anodewhich is biased at a higher positive potential then the cathode via thetarget power supply 40. The electrons that are used to ionize the samplevapor or gas are confined to a relatively small volume at a ion exitslit 4 of the apparatus. In this manner, the sample which is injectedvia sample injector 42 and introduced via the sample line 31 is directedto the ion exit slit where the electrons emitted from the ion sourcebombard the sample molecules thus creating charged ions. The ions at theion exit slit 41 are passed through a linear accelerator 18 whichcomprises a plurality of plates. The plates are biased via a voltagedividing network 43. The ion linear accelerator 18 serves to pro vide anessentially parallel beam at its exit plane. This.

is the optimum beam which the electrostatic quadrapole lens requires atits input fiducial plane. In this beam, all ions possess relativelyconstant energy. The monoenergetic beam passes through the electrostaticquadrapole lens assembly which is biased by means of the voltage dividershown in module 45. As indicated, the electrostatic quadrapole isarranged to provide a wedge-shaped beam. This shape serves to provide amatch at the entrance slit 16 of the energy analyzer and because of thewedge-shaped configuration serves to provide an optimum current densityat the s1itl6 with non-critical focusing requirements. The electrostaticquadrapole doublet enables one to obtain a wedgeshaped beam with a verylong drift length. The drift length is shown on the diagram and this isthe area where ions are accelerated along a sufficient length to givethe precursor ions time to decompose.

Basically, the formation of daughter ions involve the followingoperation. As indicated, certain ions will exhibit metastabledecompositions. The probability of such decomposition is enhanced whenthe drift time of an ion along its trajectory is increased. The drifttime, of course, is dependent upon the length of the drift path. Ametastable decomposition results in a charged daughter particle and anuncharged particle. The ions enter the entrance slit 16 of the energyanalyzer and due to theelectric field provided by the scanner 50 arecaused to focusat the output slit or aperture 17. The scanner 50provides a linear ramp up to 4000 volts to the' energy analyzer. Thisallows scanning of daughter ions to 2E with a 2,000 ev beam or to IEwith a 4,000 ev beam. The scanner is a closed loop system employing avacuum tube or transistor arranged in a sawtooth oscillatorconfiguration. The scanner is synchronized via a marker generator whichalso controls or synchronizes the display means 70, so that the abscissaof the displayed energy scan is ascertained. It was found that theseparation between the entrance slit l6 and the collector slit 17 couldbe about 4 inches to provide optimum collection of daughter ions. Thechoice of the slit width isprimarily a function of the desiredresolution. Resolution is determined by the effective width of theentrance and collector or exit slits as well as the beam energy. Forpurposes of ion kinetic energy analysis, it has been found that with abeam energy of about 3,000 electron volts, one can adequately obtainresolutions of 200 to 300 with normal size apertures. Since the spectra,unlike mass spectra, display a wide variation of peak widths, one doesnot require very high resolution as it has been found that the pluralityof peaks result in a unique fingerprint of the sample to be detected.For example, in a system using the apparatus shown above, it waspossible to pick out and distinquish pure neopentane iso-pentane, normalpentane and cyclopentane. The differences betweeen these pentanes cannotbe easily obtained by anaylsis through a mass spectrometer while the ionkinetic energy spectrometer distinquishes the same relatively easily.

It is, of course, known that the higher the beam energy, the moreresolution one can obtain. In regard to certain isomers, such asN-butane, resolutions greater than 100 were obtained for beam energiesof 900 electron volts. Experimental results indicate that for mostsamples of interest, the sharpest line width will range from 2.0 to 10.0electron volts with many wider peaks also apparent. Unlike a typicalexpensive mass spectrometer, the output daughter ion beam available atthe aperture 17 is directed to the input of an electronic amplifier 60or an electrometer. The collecting electrode 62 is referred to as aFaraday cup. The input of the Faraday cup .is coupled to the inputelectrode of the electrometer 60. Bascially, the electrometer. 60comprises two high gain operational amplifiers arranged in a cascadeconfiguration with a shielded feedback resistor 61 arranged from inputto output. The input stage for the electrometer comprises a Mosfet stagehaving a bootstrap circuit configuration between the source and thedrain electrode. It has been found that the amplifier noise which shouldbe low is a function of input capacity. Therefore, this factor should beacknowledged in the design of the amplifier section. Input capacitanceis substantially reduced by the use of conventional boot strapping andcapacitance neutralization techniques. Such techniques are known in theart. The output of the amplifier 60 is coupled to a suitable displaymeans which may be a recorder or an oscilloscope to provide the operatorwith a spectrum characteristic of metastable decompositions.

The above description is concentrated on the use of an energy analyzer20 having a 45 beam incident angle. This type of analyzer is easy tofabricate as the entrance and exit slits are located on the ground planeand hence form part of the analyzer. In any event, it is also possibleto use an energy analyzer section with a beam incident angle of 30. Thisanalyzer offers certain advantages as will be seen.

As shown in FIG. 1, only a converging beam 30 can be used in the driftspace because of the fact that the entrance slit (8,) l6 and the exitslit (S 17 are located on the ground plane 76 of the analyzer 20.

When one uses an energy analyzer 80 (FIG. 2) with a 30 angle between thebeam and the analyzer, the entrance slit (S,) 81 and the exit slit (S 82can be located in a field-free space and remote from the ground plane ofthe analyzer. This allows the slits (S and S to be positionedperpendicular to the beam axis which is the optimum theoretical positionas this position minimizes ion scattering. The plates including theslits are mounted by means of support brackets or holders within thebeam path at a desired location.

In FIG. 2 the location of the drift space is shown between an entranceslit (5,) and the analyzer, and the beam is diverging before it entersthe analyzer and within the drift space. The slot in the analyzer islarge and hence this diverging beam enhances the probability ofacceptance of more metastable ions by the optical system. It is seenthat this is so as the aperture in the analyzer of FIG. 1 (45 analyzer)is also the entrance slit 1)- In FIG. 3 there is shown a converging beamin the drift space. This is similar to the 45 case, with the exceptionthat the aperture in the analyzer can again be larger. Here the driftspace is located between the lens and the entrance slit (5,).

Thus, the advantages of the 30 analyzer include:

I. Location of analyzing slits, entrance and exit slits, in a field-freespace, allowing the slits to be positioned perpendicular to the beamaxis.

2. The object distance can be adjusted to be either large to inches) orsmall (less than 1 inch) so that the drift space can accommodate eithera converging or diverging beam.

This enables one to select a drift space either:

1. Between the quadrapole lens and the entrance slit (8,) as in FIG. 3,or

2. Between the entrance slit (S and the energy analyzer (FlG. 2).

The increased width, due to the energy analyzer input arrangement (FIG.2), also allows the analyzer to collect or accept more metastable ionsconsidering the random probability of energy change and direction changewhich is inherent in the production of metastable. This energyanddirection change is the result of the decomposition of a precursorcursor ion. The decomposition or explosion causes an impluse of momentumto be imparted'to the daughter ion which can then proceed in anydirection according to a random probability at a new energy.

I claim:

ll. Apparatus for analyzing metastable decompositions caused by theionization of certain sample ions which decompose into daughter ions anduncharged particles comprising:

a. an ion source for bombarding an injected sample material to form aplurality of ions including metastable ions,

b. lens means responsive to said ions for forming them into a relativelymonoenergetic ion beam having a wedge-shaped configuration,

c. a scanable energy analyzer separated from said means by a givendistance selected to permit the decomposition of said metastable ionsinto said daughter ions and uncharged particles, said analyzer includingan input aperture for receiving said ion beam and an output aperture fordischarging selected ions, and

d. means coupled to said analyzer for collection of daughter ions atsaid output aperture.

2. The apparatus according to claim 1 wherein a. said lens meansincludes an electrostatic quadrapole doublet lens assembly.

3. The apparatus according to claim 1 wherein said ion source includes aPierce-type cathode for emitting electrons and a target electrode fordirecting said emitted electrons toward an ion exit slit and meanslocated proximate said exit slit and adapted to receive a samplematerial to cause said material to be bombarded by said electrons.

4. The apparatus according to claim 3 further including a plurality ofbiased accelerating plates disposed about said ion exit slit anddirected along said ion beam path for forming a parallel beam.

5. The apparatus according to claim 1 wherein said given distance isgreater than 8 inches.

6. Apparatus for analyzing metastable decompositions, comprising,

a. an ion source adapted to receive a sample material for bombarding thesame with electrons to provide a plurality of ions at an output of saidion source,

b. means including an electrostatic quadrapole lens assembly positionedproximate to said ion source output to form said ions into a beam havinga wedge-shaped configuration,

0. an energy analyzer having an input slit and an output slit andpositioned a given length from said means, said length selected topermit certain precursor ions to decompose into daughter ions anduncharged particles, said analyzer positioned so that said ion beam isdirected through said input slit,

d. biasing means coupled to said analyzer for causing daughter ions topropagate through said output slit, and

e. means responsive to said propagated daughter ions to provide anindication of their presence.

7. The apparatus according to claim 6 wherein said energy analyzer is aparallel plate energy analyzer disposed at an acute angle with respectto said ion beam.

8. The apparatus according to claim 6 wherein said energy analyzer is aparallel plate analyzer disposed at an angle of 30 with respect to saidion beam.

9. The apparatus according to claim 6 wherein said means responsive tosaid propagated daughter ion, includes,

a. a Faraday cup coupled to the input of a high gain operationalamplifier for collecting and amplifying said propagated daughter ions.

10. The apparatus according to claim 6 wherein said means including anelectrostatic quadrapole lens assembly further includes a linear plateaccelerator for focusing said beam prior to said energy analyzer.

1. Apparatus for analyzing metastable decompositions caused by theionization of certain sample ions which decompose into daughter ions anduncharged particles comprising: a. an ion source for bombarding aninjected sample material to form a plurality of ions includingmetastable ions, b. lens means responsive to said ions for forming theminto a relatively monoenergetic ion beam having a wedge-shapedconfiguration, c. a scanable energy analyzer separated from said meansby a given distance selected to permit the decomposition of saidmetastable ions into said daughter ions and uncharged particles, saidanalyzer including an input aperture for receiving said ion beam and anoutput aperture for discharging selected ions, and d. means coupled tosaid analyzer for collection of daughter ions at said output aperture.2. The apparatus according to claim 1 wherein a. said lens meansincludes an electrostatic quadrapole doublet lens assembly.
 3. Theapparatus according to claim 1 wherein said ion source includes aPierce-type cathode for emitting electrons and a target electrode fordirecting said emitted electrons toward an ion exit slit and meanslocated proximate said exit slit and adapted to receive a samplematerial to cause said material to be bombarded by said electrons. 4.The apparatus according to claim 3 further including a plurality ofbiased accelerating plates disposed about said ion exit slit anddirected along said ion beam path for forming a parallel beam.
 5. Theapparatus according to claim 1 wherein said given distance is greaterthan 8 inches.
 6. Apparatus for analyzing metastable decompositions,comprising, a. an ion source adapted to receive a sample material forbombarding the same with electrons to provide a plurality of ions at anoutput of said ion source, b. means including an electrostaticquadrapole lens assembly positioned proximate to said ion source outputto form said ions into a beam having a wedge-shaped configuration, c. anEnergy analyzer having an input slit and an output slit and positioned agiven length from said means, said length selected to permit certainprecursor ions to decompose into daughter ions and uncharged particles,said analyzer positioned so that said ion beam is directed through saidinput slit, d. biasing means coupled to said analyzer for causingdaughter ions to propagate through said output slit, and e. meansresponsive to said propagated daughter ions to provide an indication oftheir presence.
 7. The apparatus according to claim 6 wherein saidenergy analyzer is a parallel plate energy analyzer disposed at an acuteangle with respect to said ion beam.
 8. The apparatus according to claim6 wherein said energy analyzer is a parallel plate analyzer disposed atan angle of 30* with respect to said ion beam.
 9. The apparatusaccording to claim 6 wherein said means responsive to said propagateddaughter ion, includes, a. a Faraday cup coupled to the input of a highgain operational amplifier for collecting and amplifying said propagateddaughter ions.
 10. The apparatus according to claim 6 wherein said meansincluding an electrostatic quadrapole lens assembly further includes alinear plate accelerator for focusing said beam prior to said energyanalyzer.